Flexible printed circuit boards having a three-layered structure of insulating substrate/adhesive/metal foil, using as the adhesive an epoxy, acrylic, polyamide, phenol adhesive or the like, are well known. In these flexible printed circuit boards, since the heat resistance and adhesion of the metal layer are determined by the properties of the adhesive, they have a problem in heat resistance. Flexible printed circuit boards wherein a precursor of a thermoplastic polyimide is used as an adhesive for increasing heat resistance, and wherein a metal foil is heat-pressed at a high temperature, are known (Patent Literatures 1 and 2). However, these flexible printed circuit boards have a problem of residual strain after processing because the metal foil needs to be heat-pressed at a high temperature, and a problem in that patterning with a small pitch is difficult because the thickness of the metal foil used for the heat-press is usually not less than 10 μm.
Flexible printed circuit boards having a bilayered structure produced by directly forming a metal layer by sputtering or electroless plating on an insulating film, for example, a non-thermoplastic polyimide film or aramide film, and then forming a metal layer by electrolytic plating are known. These flexible printed circuit boards have a drawback in that adhesion is poor, especially decrease in adhesion after heat load is large.
Flexible printed circuit boards produced by forming a thermoplastic polyimide layer on a non-thermoplastic polyimide, sputtering copper on the surface of the thermoplastic polyimide layer, and forming a metal layer by electrolytic plating are also known (e.g., Patent Literature 3). However, these flexible printed circuit boards have a problem in that the adhesion of fine wirings is poor.    Patent Literature 1: JP 4-146690 A    Patent Literature 2: JP 2000-167980 A    Patent Literature 3: JP 2003-251773 A    Patent Literature 4: WO99/19771